1. Field of the Invention
This invention relates generally to the structure and manufacture of electric motors and, more specifically, to improved magnet rotor assemblies.
2. Background
Magnet rotors have gained acceptance in many applications including computer storage devices for media formats such as HOD, FDD, CD-ROM and DVD; automotive applications such as sensors, motors, power steering; office automation devices such as facsimile machines, copiers, scanners, and printers; and consumer electronics devices such as cameras and stereo components, hand-held power tools and the like. Neodymium-iron-boron (NdFeB) magnetic powders have emerged as a preferred magnet material for use in making magnet rotors for such applications. However, other useful materials for these applications include ferrite, samarium cobalt, AlNiCo and mixtures thereof. The selection of the particular magnetic materials used to form magnet rotors is often a function of such considerations as temperature, environment and other application conditions and on the desired performance of a magnet rotor under these conditions.
A magnet rotor can be made by injection molding techniques or can be assembled using compression molded, extruded, or sintered pre-formed magnets. When a magnet rotor is made using injection molding techniques, the magnet material is applied to a rotor shaft in a viscous, fluid state. Upon curing, the magnet material adheres to the rotor shaft. When a magnet rotor is made using a pre-formed magnet, a magnet sleeve is first shaped by, for example, compression molding, extruding or sintering. The magnet sleeve normally is positioned over the rotor shaft and fixed in place with an adhesive. In most such constructions the rotor shaft includes a core that has been press fit onto a shaft, thus increasing the diameter of the rotor shaft to meet specific requirements.
The magnetic properties of injection molded magnets are normally lower than those of compression molded magnets. Injection molded magnets typically contain about 65 to 70 volume percent magnet material compared to about 80% for typical compression molded, magnets resulting in a magnetic powder density of about 5.2 g/cm3 in injection molded magnets and about 6.1 g/cm3 in compression molded, magnets. This difference in magnetic density results in a remanence of 0.52 Tesla for magnet rotors made with injection molded magnets and of 0.68 Tesla for magnet rotors made with such pre-formed magnets. The energy product for magnet rotors made with such injection molded magnets is 51 kJ/cm3 compared with 75 kJ/cm3 for magnet rotors made with such pre-formed magnets.
Although magnet rotors made with pre-formed magnets have stronger magnetic properties and better torque delivery to the shaft, there have been drawbacks to their use in certain applications. It has been a problem that magnet rotors made with such pre-formed magnets tend to have an undesirably high failure rate when operated in a high frequency reciprocating mode, especially at high temperatures. It also is a problem that magnet rotors made using pre-formed magnets fixed to a shaft or core by an adhesive often exhibit an undesirably high run-out (0.3 to 0.4 mm) and noticeable cogging torque. It is thought that these drawbacks are related mainly to lack of uniformity in the thickness of the adhesive layer that bonds the pre-formed magnet to the rotor shaft. One typical adhesive material used to attach the pre-formed magnet sleeves to rotor shafts attains its maximum bonding strength at a thickness of about 5 to about 25 microns and exhibits weaker bonding when the adhesive layer is either thicker or thinner. (The thickness providing maximum bonding strength will, of course, vary from adhesive to adhesive.) Previously known manufacturing methods have not been able to mass produce magnet rotors in which pre-formed magnet sleeves are attached to rotor shafts with adhesive layers that are mostly at a desired uniform thickness to attain maximum adhesive strength for the particular adhesive used. As a result of the rotor shafts not being centered in the magnet sleeves the adhesive layers have thick and thin regions, both of which will result in weaker bonding. High frequency reciprocating operation of such prior art magnet rotors, especially at high temperatures, can cause early failure. Also, regions in the adhesive layer that are thicker than desired are known to result in higher run-out and cogging torque. In the past, it also has been a problem that during high temperature curing of an adhesive layer between a magnet sleeve and a rotor shaft, air bubbles from the porous magnet sleeve tend to enter the adhesive layer, resulting in areas of weaker bonding.
There is a need for:
A. improved magnet rotors that demonstrate reduced failure during high frequency reciprocating operation, especially at elevated temperatures.
B. There also is a need for improved magnet rotors that show reduced run-out during high speed operation.
C. Additionally there is a need for improved magnet rotors that demonstrate improved break-away torque and increased resistance to slippage during high frequency reciprocating operation.
D. There also is a need for improved magnet rotors having reduced cogging torque.
E. There is a need for a method for making magnet rotors that enables manufacturers to take advantage of the greater magnetic strength of pre-formed magnets.
F. Further, a need exists for an improved magnet rotor that retains its physical strength at high operating temperatures and a method for making such an improved magnet rotor.
G. Additionally there is a need for motors comprising such improved rotors.